Although morbidity and mortality data consistently demonstrate that older individuals are at greater risk in hot environments, few investigators have studied the relationship between age and thermoregulatory response. It is unclear to what extent decreased heat tolerance in the elderly is a function of decreased aerobic capacity, as opposed to age-related changes in neural function, functional capacity of the sweat gland, etc. Statistically, more older women than men are victims of heat- related illness. Yet it is equally unclear whether this simply reflects the relatively larger number of women in this population, or whether a sex-difference exists in thermoregulatory function of older adults. Previous investigations in this area have suffered from the inability to match groups of older and younger subjects with respect to aerobic capacity (Drinkwater et al., 1982). We have alleviated that problem by recruiting a subject population of highly fit men and women over the age of 55 which we propose to compare with (1) younger men and women of similar aerobic fitness, (2) men and women with lower fitness levels, and (3) very fit younger men and women. This investigation proposed to systematically compare physiological effector (body temperature, cardiovascular, and sudomotor) responses to hyperthermia among these groups. Subjects will be tested at rest and during moderate exercise in 3 environments (neutral, hot-dry, and warm-humid) before and after heat acclimation. The specific aims of this research include the following comparisons among the aforementioned groups. 1. onset characteristics (temporal and temperature thresholds) of effector responses (sweating and cutaneous vasodilation); 2. body temperature, heart rate, blood pressure, and sweating responses to moderate exercise in the 3 environments; 3. differences in heat-activated sweat gland (HASG) number and density, sweat production per gland, and HASG dose-response characteristics relative to a neurotransmitter analog (methylcholine); 4. percentage changes in blood volume, red cell volume, and plasma volume due to exercise in these environments, and 5. alterations in magnitude and control of cutaneous blood flow and cardiac output.